Measuring execution time of a function in C++

Question

I want to find out how much time a certain function takes in my C++ program to execute on Linux. Afterwards, I want to make a speed comparison . I saw sever

Answer 1

In Scott Meyers book I found an example of universal generic lambda expression that can be used to measure function execution time. (C++14)

auto timeFuncInvocation = 
    [](auto&& func, auto&&... params) {
        // get time before function invocation
        const auto& start = std::chrono::high_resolution_clock::now();
        // function invocation using perfect forwarding
        std::forward(func)(std::forward(params)...);
        // get time after function invocation
        const auto& stop = std::chrono::high_resolution_clock::now();
        return stop - start;
     };

The problem is that you are measure only one execution so the results can be very differ. To get a reliable result you should measure a large number of execution. According to Andrei Alexandrescu lecture at code::dive 2015 conference - Writing Fast Code I:

Measured time: tm = t + tq + tn + to

where:

tm - measured (observed) time

t - the actual time of interest

tq - time added by quantization noise

tn - time added by various sources of noise

to - overhead time (measuring, looping, calling functions)

According to what he said later in the lecture, you should take a minimum of this large number of execution as your result. I encourage you to look at the lecture in which he explains why.

Also there is a very good library from google - https://github.com/google/benchmark. This library is very simple to use and powerful. You can checkout some lectures of Chandler Carruth on youtube where he is using this library in practice. For example CppCon 2017: Chandler Carruth “Going Nowhere Faster”;

Example usage:

#include 
#include 
#include 
auto timeFuncInvocation = 
    [](auto&& func, auto&&... params) {
        // get time before function invocation
        const auto& start = high_resolution_clock::now();
        // function invocation using perfect forwarding
        for(auto i = 0; i < 100000/*largeNumber*/; ++i) {
            std::forward(func)(std::forward(params)...);
        }
        // get time after function invocation
        const auto& stop = high_resolution_clock::now();
        return (stop - start)/100000/*largeNumber*/;
     };

void f(std::vector& vec) {
    vec.push_back(1);
}

void f2(std::vector& vec) {
    vec.emplace_back(1);
}
int main()
{
    std::vector vec;
    std::vector vec2;
    std::cout << timeFuncInvocation(f, vec).count() << std::endl;
    std::cout << timeFuncInvocation(f2, vec2).count() << std::endl;
    std::vector vec3;
    vec3.reserve(100000);
    std::vector vec4;
    vec4.reserve(100000);
    std::cout << timeFuncInvocation(f, vec3).count() << std::endl;
    std::cout << timeFuncInvocation(f2, vec4).count() << std::endl;
    return 0;
}

EDIT: Ofcourse you always need to remember that your compiler can optimize something out or not. Tools like perf can be useful in such cases.